As semiconductor integrated circuits have become finer and more highly integrated, the steps of semiconductor manufacturing processes have increased in number and become more complicated. As a result, the surfaces of semiconductor devices are no longer always flat. The presence of steps on the surfaces of semiconductor devices leads to step interruptions of wiring and local increases in resistance, etc., and may thus cause disconnections and drops in the electrical capacity. Furthermore, in insulating films, such steps lead to a deterioration in the withstand voltage and leakage.
Meanwhile, as semiconductor integrated circuits have become finer and more highly integrated, the wavelengths of the light sources of semiconductor exposure apparatuses used in photolithography have become shorter, and the numerical aperture values or so-called NA values of the projection lenses of such semiconductor exposure apparatuses have become larger. As a result, the focal depths of the projection lenses of such semiconductor exposure apparatuses have become substantially shallower. In order to handle such increasing shallowness of the focal depth, flattening of the surfaces of semiconductor devices is required to a greater degree than in the past.
To give a concrete example, a flattening technique such as that shown in FIG. 7 has become essential in semiconductor processes. Here, a semiconductor device 14, an inter-layer insulating film 12 consisting of SiO2 and a metal film 13 consisting of Al are formed on the surface of a wafer 11. FIG. 7(a) shows an example of the flattening of the inter-layer insulating film 12 on the surface of the semiconductor device. FIG. 7(b) shows an example of the formation of a so-called damascene by the polishing of the metal film 13 on the surface of the semiconductor device.
A chemical mechanical polishing or chemical mechanical planarization (hereafter referred to as “CMP”) technique is widely used as a method for flattening the surface of such a semiconductor device. Currently, this CMP technique is the only method that can flatten the entire surface of a wafer.
CMP has been developed on the basis of wafer mirror surface polishing methods. FIG. 8 is a schematic structural diagram of the polishing (flattening) apparatus used in CMP. The polishing apparatus is constructed from a polishing member 15, a polishing object holding part (hereafter also referred to as a “polishing head”) 16 and a polishing agent supply part 18. Furthermore, a wafer 17 that constitutes the object of polishing is attached to the polishing head 16, and the polishing agent supply part 18 supplies a polishing agent (slurry) 19. The polishing member 15 is a member in which a polishing body (hereafter also referred to as a “polishing pad”) 21 is bonded to the surface of a platen 20.
The wafer 17 is held by the polishing head 16, and is caused to swing while being rotated; furthermore, this wafer 17 is pressed against the polishing body 21 of the polishing member 15 with a specified pressure. The polishing member 15 is also caused to rotate, and is thus caused to perform a relative motion with respect to the wafer 17. In this state, the polishing agent 19 is supplied to the surface of the polishing body 21 from the polishing agent supply part 18; the polishing agent 19 diffuses over the surface of the polishing body 21, and enters the space between the polishing body 21 and the wafer 17 as relative motion takes place between the polishing member 15 and wafer 17, so that the surface of the wafer 17 that is to be polished is polished. Specifically, mechanical polishing by the relative motion of the polishing member 15 and wafer 17 and the chemical action of the polishing agent 19 act in a synergistic manner, so that favorable polishing is performed.
FIG. 9 is a schematic diagram which shows another polishing apparatus. In this polishing apparatus, the polishing head 16 is on the lower side, and the wafer 17 is chucked above this polishing head 16. Furthermore, the polishing body 21 has a smaller diameter than the wafer 17, and is bonded to a polishing platen 20 that is disposed above. Specifically, the polishing body 21 is caused to swing while being rotated together with the polishing platen 20, and is pressed against the wafer 17 with a specified pressure. The polishing head 16 and wafer 17 are also caused to rotate, and are thus caused to perform a relative motion with respect to the polishing body 21. In this state, a polishing agent 19 is supplied to the surface of the wafer 17 from the polishing agent supply part 18; the polishing agent 19 diffuses over the surface of the wafer 17 and enters the space between the polishing body 21 and the wafer 17 as relative rotation occurs between the polishing member 15 and the wafer 17, so that the surface of the wafer 17 that is to be polished is polished.
However, the number of types of wafers that require polishing is extremely large, and independent polishing conditions (recipes) that are suited to these respective types of wafers must be set.
For example, in the case of polishing that extends over a plurality of layer structures such as Cu damascene, Cu is ordinarily polished in a primary polishing process, and Ta is polished in a secondary polishing process. In this case, the uniformity varies greatly even under the same polishing conditions, as a result of differences in the polishing agent and object of polishing.
Accordingly, this method is troublesome in that polishing conditions must be separately prepared for each polishing operation. Furthermore, in the case of metal polishing, an oxidizing agent such as hydrogen peroxide must be added in addition to the polishing agent. Since the polishing profile varies according to the amount of this additive even in the case of the same polishing agent, the polishing conditions must be varied for all cases when the type of polishing agent, additive and object of polishing vary.
Polishing conditions include the type of polishing liquid, the type of polishing pad, the rotational speed of the polishing head and polishing member, the swinging speed of the polishing head, and the pressing pressure of the polishing head, etc. The rotational speed of the polishing head and polishing member, the swinging speed of the polishing head and the pressing pressure of the polishing head are functions of time or functions of polishing head position.
Conventionally, as the method used to set the polishing conditions in accordance with the type of wafer involved, a method has been employed in which test polishing is performed by trial and error on the basis of experience, and polishing conditions that produce the desired treated shape are found. In this case, numerous wafers are used in this test polishing, and considerable time is spent in determining the polishing conditions.
Furthermore, even assuming that the type of wafer used can be specified, and that standard polishing conditions can be found, the pre-polishing surface shape of the wafer that is actually polished varies according to the manufacturing lot. Therefore, for each manufacturing lot, fine adjustment of the polishing conditions must be performed by performing further test polishing. However, even if fine adjustment for each manufacturing lot is thus performed, a problem remains: namely, variation within manufacturing lots cannot be handled.
In conventional polishing apparatuses, in which the polishing body is larger than the wafer that is being polished, the following problem arises: namely, the size of the apparatus itself is increased as the diameter of the wafer increases. Furthermore, the following drawback is also encountered: specifically, the replacement work of consumable parts that require replacement such as the polishing pad is extremely difficult as a result of this large size. Moreover, in cases where there are indentations or projections arising from irregularities in the film on the surface of the wafer prior to polishing, it is extremely difficult to polish the wafer to a flat surface by appropriately dealing with these indentations and projections. Furthermore, in the case of wafers in which the initial film thickness and shape constitute an M type or W type, etc., according to the film formation process, there may be instances in which it is necessary to polish the remaining film to a uniform shape. It is difficult to meet such requirements in the case of a conventional polishing apparatus.
Recently, polishing apparatuses using a polishing body that is smaller than the wafer that is being polished (as shown in FIG. 9) have been developed and used as polishing apparatuses that solve these problems. Since such polishing apparatuses have a small polishing body, these apparatuses are advantageous in that the size of the polishing part in the polishing apparatus can be reduced. Furthermore, with regard to the replacement of consumable parts as well, such replacement work is extremely easy because of the small size of the parts.
Furthermore, in such polishing apparatuses using a polishing body that is smaller than the wafer that is being polished, the polishing profile can be freely varied by varying the probability of the presence of the polishing body on respective parts of the wafer. Accordingly, it is possible to handle cases in which there are indentations and projections in the surface of the wafer prior to polishing.
However, the fact that such fine adjustment is possible means that the polishing conditions must be determined with greater precision. Specifically, the types of polishing conditions increase in number and become more complicated, and there is an increase in the number of times that polishing conditions must be determined; furthermore, there is an increase in the quantity of time and wafers required in order to determine a single polishing condition. Moreover, even in cases where fine adjustment is not required, since the polishing body is small, the fact that the polishing conditions are complicated compared to those of a polishing apparatus using a conventional large polishing body remains unchanged.
Specifically, in the case of polishing using a small-diameter pad, it is necessary to apply variable-speed swinging (besides rotation) in order to vary the probability of the pad being present on the wafer surface, and to perform load control that reduces the load in order to suppress a rise in the polishing speed at the edges of the wafer. Accordingly, as a result of the addition of these control actions, the polishing conditions are greatly increased in complexity.
Thus, a method in which the polishing conditions are determined by simulation has been developed as one means of solving the problem of a considerable time being required for the determination of the polishing conditions. However, in the polishing process, the polishing body undergoes elastic deformation; furthermore, the flow of the polishing agent between the polishing body and the object of polishing is complicated, and frictional heat is generated during polishing. Accordingly, the expression of the overall polishing process in terms of numerical formulae is difficult, and an all-purpose numerical model has not yet been obtained.